When hanging motionless in a seat harness or sitting in a harness for a long period of time, two conditions can occur: harness syndrome or compression or crush syndrome.
The first, harness syndrome, was originally called hypothermic exhaustion until studies found the cause as the simple act of sitting in a harness. The reason for the study, which was done in France, was partly due to the 15 deaths of cavers in a short amount of time. This study can be found in the French Speleo Magazine - I believe it was done in '91 or '93. Considerable difficulties (blood pressure rose abnormally high, faintness, rapid pulse, diaphoretic and breathlessness) occurred within 15 minutes of hanging.
The second condition which might occur is compression or crush syndrome. Originally, this syndrome was associated with injuries that dealt with extremities being trapped (BFR's, cars, machinery...) for a period of time. The person is fine, they might have a leg trapped under a rock which is impeding the circulation within that leg. While the circulation is not occurring in the trapped leg, toxins (waste by-products of the cells) start to build up within the vessels of the trapped leg, below the compression. The body is always dealing with low, very low concentrations of these toxins without any ill-effects. However, when a crushed extremity (i.e., the trapped leg) is released, the trapped toxins suddenly flow into the circulatory system and shocks the system. This will kill in seconds if not prepared - IV's a must prior to the release. However, this syndrome won't occur in seconds, but an hour or two. This also depends on how large the area is affected and if the circulation is completely or partially shutdown. Hanging in a harness, motionless, for long period of time will produce the same conditions in the legs as if it was trapped under a rock.
Harness hang is a life-threatening situation. A person unconscious, on-rope is in grave danger and must get off immediately. Usually, harness hang will kill them, before the compartmental or crush syndrome will. The best treatment for harness hang is to get the person off rope and on the ground.
A caver experiencing difficulty on-rope, due to exhaustion or technical problems, must have assistance immediately before the situation turns into a harness hang problem. A caver hanging completely inert (head back, arms out) must be unhooked quickly by other team members. This person will not survive 30 min. A caver should not be allowed to ascend alone, even if they happen to be in very good shape. (pay attention to all team members - accountability in health and location. A tired caver should refuse or not be allowed to begin a long or difficult ascent, especially in a wet cave, with out a recovery rest period. All vertical cavers must be more than proficient in pick-offs. If on-rope for some reason, move - this keeps the muscles moving, which in turn helps in circulating the blood.